Atlantic Technology AT-1 loudspeaker

John Atkinson nudged my ribs with an elbow. "Did you bring your Cornelius CD with you?" he whispered.

It was the 2010 Consumer Electronics Show, and JA and I were nearing the end of a dog-and-pony act expertly presented by Atlantic Technology's president, Peter Tribeman, touting a prototype of his company's new loudspeaker, the AT-1. JA and I had just heard about the finer points of the AT-1's new bass-venting technology, the Hybrid-Pressure Acceleration System (H-PAS), which was supposed to combine all the benefits and qualities of a transmission-line enclosure, horn loading, and sealed and ported designs. At the time, I didn't care if it combined all of the qualities of Kim Kardashian, Sacagawea, Joan of Arc, and Marie CurieI was just thrilled that the AT-1s were sounding so good in a partitioned ballroom.

Another elbow to the ribs: "I want to hear those kick drums from 'Fit Song' on this system," JA muttered.

To avoid more bruising to my midsection, I politely raised my hand, asked if we could hear a track from my CD, and was kindly obliged. Tribeman even handed me the remote control. I turned the system up to levels you're not really supposed to reach at a hi-fi show.

The room went quiet as the unassuming black towers of the AT-1s took on "Fit Song." Each kick-drum sample hit the assembled crowd in the gut with a speed and a weight I hadn't heard in that entire CES. Jaws dropped all over the room, at the sound of the AT-1s and at how frickin' awesome this music is. At the song's abrupt end, the listeners almost applauded. When Tribeman told us that we were listening to a speaker that would cost somewhere between $2000 and $3000/pair, I knew I had to get review samples as soon as the model was put into production.

H-PAS
The technology inside the AT-1's H-PAS enclosure isn't new, but its application is. I asked Peter Tribeman about the enclosure's pedigree and development:

"H-PAS is based upon earlier work originally introduced by acoustician Philip Clements. In those days Phil was looking for ways to acoustically eliminate some distortion byproducts for his newly developed vented speaker designs. He discovered by accident that as he adjusted the relative lengths and angles of his internal chambers with small changes, the bass extension and intensity of the low-frequency output from the vent was dramatically improved. Phil realized at that point that he could achieve incredible bass response with high efficiency and low distortion in 'normal'-size speaker cabinets. By trial and error and constant experiments, Phil made up the rules of this newly discovered technology. Three decades later, we approached Phil with some proposals as to how to make some of those designs in a much smaller footprint. Both Atlantic and Phil brought computers, mathematicians, and sophisticated algorithms to the table to apply up-to-date science to solve the mysteries of his early work in speaker fluid dynamics. From this collaborative effort came what is now known as H-PAS, and our first product with this technology, the AT-1."

The AT-1 looks like a typical two-way, floorstanding speaker with a 1.1" silk-dome tweeter nestled between two 5.25" woofers with graphite-loaded homopolymer cones. Inside, however, things look a bit unusual. Glance at the cutaway diagram provided by Atlantic Technology and you get the impression that the AT-1 looks essentially like a modified transmission-line design. But according to Steve Feinstein, Atlantic's director of marketing and product development, that appearance is misleading:

"Because of the way the internal chambers work, the AT-1 really combines the essential operational characteristics of four different speaker technologies into one product: The top chamber exerts the driver control on the woofers' cones like a sealed design; the long, multi-segment internal chamber, which becomes narrower and narrower, is like an inverted horn, increasing sensitivity; the internal 'bass trap' portion, which literally 'peels off' the bass wave's harmonic-distortion products (based on that chamber and its opening's dimensional relationship to the harmonic frequencies in the bass region), is transmission line-like in its effect; and, finally, the vent opening, which acts like a horn (because of the internal wave-compressing 'plates' that are employed at that point) and also like a reflex vent. We're getting the benefits of all four types."

A benefit of the H-PAS topology is that it allows the designer to use smaller, quicker woofers without sacrificing low-bass extension. The AT-1's 3dB bass-rolloff point is specified at a low 29Hzpretty darned good for a pair of 5.25" cones. The speaker also boasts a sensitivity of 89dB, which is nearly unheard of for small drivers capable of such low bass.

The AT-1's woofers are crossed over to its 1.1" silk-dome tweeter at 2kHz, the tweeter seeing a third-order slope and the woofer a second-order handoff. While many two-way designs cross over at higher frequencies, the AT-1's 2kHz crossover ensures a more even dispersion pattern at the top of the woofers' passband and better integration with the tweeter. However, the lower the crossover frequency, the more energy a small tweeter is asked to produce, and the greater chance it has of overheating or running out of excursion capability.

The above speaker is a decent performer - for a bass reflex. However, there is nothing unique, patent worthy, or even new about the design. PMC has been using resonance traps in its speakers for years.The "patent" shows a tapered transmission line shape with an anti resonance trap whose opening is located at a pinch point within the line. The distortion measurements shown in the patent are as misleading as the marketing hype for this speaker - distortion figures are presented only for select frequencies to make it look better on paper than it really is. In fact, the impedance plot shows the telltale sign of an anti resonance trap - a impedance peak around 100hz. This is actually not a good thing. If you were to measure THD at that frequency, you'd find a noticeable increase. "Anti resonance traps" or acoustic filters have been around since the beginning of transmission lines. Their use actually demonstrates a lack of knowledge/skill in design - not an advancement.

Moreover, the claims of mixing bass reflex with transmission line are totally bogus. True transmission lines possess a gradual 12db per octave rolloff in the bass region below the transducer's Fs or fundamental resonant frequency. This speaker clearly does not. It's rolloff is representative of a reflex design (steeper 24 db/octave roll off below transducer resonance). Compare the low frequency response of this to that of the Vivid Giya "quasi" transmission lines:

What this AT speaker is is a bass reflex with a small horn attached to the port opening and an acoustical filter incorporated to help dampen the upper resonances of a primitive design. Phil is just warming over a 25 year old transmission line patent with the application of an internal restrictor.

Despite the false claims of this speaker's "designers", there is no continuum between a true transmission line and a pseudo "mass loaded transmission line". Mass loading means bass reflex - PERIOD. The velocity of low pressure pulses escaping a transmission line should be in the neighborhood of 340 m/s. The velocity of air transfer in most reflex designs is about 18- 20 meters per second (notice I said "air transfer" - not low pressure acoustical pulses - big, big difference). The mechanical tuning of a reflex design is centered about a Helmholtz resonance - a slug of air being forced through a restrictive pipe. The time it takes to ram the air slug through the restrictor determines the tuning frequency. With a transmission line, there is not supposed to be any "restriction" beyond the slight tapering of the cavity behind the speaker diaphragm that follows the natural reduction in acoustic pressure that occurs when sound radiates a given distance from its source (the 1/distance attenuation rule). The timing (frequency tuning) of a transmission line is established by distance from the source and the frequency whose peak presssure occurs at that distance. Contrary to now unfortunately common assertions, the two approaches can't be mixed. You either have restrictive timing and the inherent 24 db/octave rolloff or you don't.The design concepts are worlds apart and so are the results - particularly when you attempt to feed both designs with sub bass programme material at high volume. The reflex design unloads (over excursion) below tuning frequency and the transmission line just keeps humming along. The frequency response and impedance plots always give them away.